1. Field of the Invention
The subject invention generally pertains to a tool for assembling threaded sucker rods of oil wells and other wells, and more specifically pertains to a switch on a rod tong that detects when the shoulder point of the rod""s threaded connection has been reached.
2. Description of Related Art
Oil wells and many other types of wells often include a sucker rod pump for pumping oil or other fluid from deep within a well bore to the surface of the earth. A sucker rod pump is a reciprocating piston/cylinder type pump situated at the bottom of a long string of tubing that conveys the pumped fluid upward to the earth""s surface. An oscillating drive at ground level is coupled to raise and lower the pump""s piston by way of long string of sucker rods that may extend over 10,000 feet through the interior of the tubing. The string of sucker rods is comprised of individual solid rods of about 0.5 to 1.125 inches in diameter and about 25 to 30 feet long. Each sucker rod has an axial shoulder and male threads at each end that allow the rods to be tightly connected end-to-end by way of female threaded rod couplings (also referred to as boxes). The couplings also serve as a wear surface that protects the more expensive sucker rod from wear as the string of sucker rods may slide up and down along the interior of the tubing for millions of cycles over its lifetime.
Properly tightening each threaded joint of a string of sucker rods is critically important, as even a single improperly tightened joint can lead to a premature separation, fatigue cracking, or complete breakage of the string. This not only interrupts the ongoing operation of the well, but repairing a string of sucker rods is very expensive, due to its inaccessibility. Usually the entire string of sucker rods is removed from the well bore to repair a single joint. For a 10,000-foot string of 25-foot sucker rods, there are about 800 threaded joints. Thus, a reliable system is needed to properly tighten every single one.
Today, power rod tongs are possibly the most common tools for assembling and disassembling a string of sucker rods. Conventional tongs, such as those provided by BJ-Hughes Machinery of Houston, Tex., includes two sets of jaws: one set being driven to rotate relative to the other. To assemble a new joint, a sucker rod is first manually screwed hand-tight into each end of a coupling. The rod tong is positioned to engage one set of tong jaws with mating flats of one sucker rod, and the other set of jaws with mating flats of the other sucker rod. This places the coupling generally between, but spaced apart from, the two sets of jaws. Actuating the tong rotates one rod relative to the other, so that both rods screw tightly into the coupling generally at the same time. As the connection tightens, the tong eventually stalls at a torque or pressure preset by the operator. When the tong stalls, the operator assumes that the connection is properly torqued with the proper preload.
Some rod tongs may shut off automatically in response to a switch, as disclosed in U.S. Pat. No. 3,768,573. In this case, a hydraulically actuated switch cuts out the drive motor shortly after the hydraulic pressure exceeds a predetermined limit. The hydraulic pressure that actuates the switch is the same pressure that drives the hydraulic motor. This poses several possibilities for error, as the hydraulic pressure driving the motor is not a reliable indicator of the actual torque applied to the sucker rods. For example, at the beginning of the workday, the switch may be prematurely tripped by abnormally high pressure caused by cold, viscous hydraulic fluid or excessive friction in the drive mechanism of the tongs. Later in the day, as the hydraulic fluid warms up with use, switch actuation may be delayed, which may cause the sucker rods to be over tightened.
To improve the accuracy of tightening rod tongs, it is an object of some embodiments of the invention to consistently identify a shoulder point of a sucker rod connection, and do so substantially independently of the tong motor""s hydraulic pressure.
A second object is to provide a rod tong with the ability to consistently identify a shoulder point of a sucker rod connection.
A third object is to provide a device for adjusting the point at which a rod tong identifies a shoulder point.
A fourth object is to provide a rod tong with a counter that accurately measures how far a first set of jaws rotates relative to a backup wrench.
A fifth object is to provide a rod tong with a pin connector that allows a backup wrench to pivot and slide relative to the tong""s housing.
A sixth object of some embodiments of the invention is to enable rod tongs to identify a shoulder point of a sucker rod connection even if the motor driving the tongs is not a hydraulic motor.
A seventh object of some embodiments of the invention is to turn off a rod tong automatically and with repeatability by doing so substantially independently of the tong motor""s hydraulic pressure.
These and other objects of the invention are provided by sucker rod tool that includes a switch that responds to movement of a backup wrench as the rod tool tightens a pair of sucker rods.